A solder which has been used from long in the past is a Sn—Pb alloy. A eutectic solder having a composition of Pb-63Sn has been much used, particularly for soldering of electronic equipment. Since this Sn-63Pb eutectic solder has good wettability and spreadability with respect to the surface of a metal to be soldered, it has the excellent advantages that it produces few defects at the time of soldering such as unsoldered portions, voids, or bridges, and that reliable soldered joints are obtained.
This solder has a eutectic composition, and it has the important advantage that its solidus temperature and liquidus temperature are the same, and each temperature, i.e., the melting point is as low as 183° C. In general, in soldering of electronic equipment, soldering is carried out by melting at a temperature of 30-50° C. above the liquidus temperature. Accordingly, when using the above-described eutectic solder, the soldering temperature is 210-230° C. At a soldering temperature of this order, soldering is carried out without thermal effects on electronic parts or printed circuit boards which are sensitive to heat. From this standpoint as well, a eutectic solder with a melting point of 183° C. has excellent reliability.
For the above reasons, a Sn-63Pb eutectic solder is extensively used in electronic equipment for soldering when mounting electronic parts on printed circuit boards or when soldering elements to substrates within electronic parts.
When electronic equipment becomes old and becomes inconvenient to use or breaks, it is often disposed of by being discarded. When electronic equipment is discarded, parts made of a single material such as frames made of metal, cases made of plastic, and displays made of glass can easily be recovered and reused as resources. However, it is difficult to reutilize printed circuit boards in which the resins in insulating materials and the metals in conductors or solder are bonded to each in a complicated shape, so they are usually disposed of by burial. If acid rain which seeps into the ground contacts printed circuit boards which have been disposed of by burial, the lead in the solder is dissolved out by the acid rain and is mixed into underground water. If underground water containing lead is drunk by humans or livestock over a long period, lead accumulates within the body, and there is the danger of lead poisoning eventually taking place. Therefore, the use of lead is being restricted on a worldwide scale, and so-called lead-free solder which does not contain lead has come to be used.
Lead-free solder is solder having Sn as a main component, to which Ag, Cu, Bi, In, Zn, Ni, Cr, P, Ge, Ga, and the like are suitably added.
Conventional lead-free solders are binary alloys such as Sn—Ag, Sn—Cu, Sn—Sb, Sn—Bi, Sn—In, and Sn—Zn having Sn as a main component, or multi-element alloys in which other elements are added to these binary alloys.
Among these, Sn—Ag based alloys and Sn—Cu based alloys are the most widely utilized lead-free solders on account of having excellent mechanical strength compared to Sn-63Pb eutectic solder. However, almost all of these lead-free solders have a liquidus temperature of at least 220° C., and the solidus temperature is 217° C. even for the tertiary eutectic composition of Sn—Ag—Cu having the lowest solidus temperature, and this is significantly higher than that of a Pb-63Sn eutectic solder. Accordingly, the soldering temperature when using a Sn—Ag based or Sn—Cu based lead-free solder becomes at least 250° C., and a deterioration in performance or thermal damage may result in electronic parts and printed circuit boards. A Sn—Sb based lead-free solder also has a high liquidus temperature of at least 240° C., and it has problems with respect to thermal effects on electronic parts and the like.
A Sn—In lead-free solder has a Sn-52In eutectic composition with a eutectic temperature at 117° C. This Sn—In lead-free solder has a lower melting point than a Pb-63Sn eutectic solder, and thermal effects on electronic parts and printed circuit boards can be even further reduced. However, if the melting point of solder is too low, it may lead to problems after soldering. For example, at the time of use of electronic equipment, coils, power transistors, resistors, and the like generate heat, and if such heat-generating parts are soldered with a solder having too low a melting point, the solder may melt due to the generated heat, or even if it does not melt, its mechanical strength decreases, and it can be easily peeled off with a small force. In addition, the amount of In which is produced is small, and it an element for which there is a possibility of its supply on earth being exhausted, so it is extremely expensive. Therefore, the addition of large amounts of In is undesirable from the standpoint of economy.
A Sn—Bi lead-free solder has a Sn-58Bi eutectic composition with a eutectic temperature of 139° C. In the same manner as the above-described Sn—In lead-free solder, this Sn—Bi lead-free solder has a melting point which is too low, and it is not suited for soldering of electronic parts which generate heat. In addition, Bi itself has the property of being extremely brittle, and a Sn—Bi lead-free solder which contains a large amount of Bi has the problem that peeling easily takes place if an impact is applied after soldering.
A Sn—Zn based lead-free solder has a Sn-9Zn eutectic composition with a eutectic temperature of 199° C. This eutectic temperature (melting point) is fairly close to the melting point (183° C.) of the above-described Pb-63Sn eutectic solder. Accordingly, when a Sn—Zn based lead-free solder is used for soldering many types of electronic equipment which have used Pb-63Sn eutectic solder, thermal problems, i.e., problems of thermal damage to electronic parts or easy peeling due to the generation of heat after soldering of heat-generating parts do not readily take place. Moreover, Zn is inexpensive, and it is an element contained in the human body and it has little adverse effect on the human body. Under these circumstances, Sn—Zn based lead-free solders have recently been studied more extensively.
With Sn—Zn based lead-free solders, the wettability of a binary Sn—Zn alloy is not sufficient, and unsoldered portions, voids, and the like occur. For this reason, from in the past, there have been many proposals of Sn—Zn based lead-free solders having improved wettability by the addition of one or more metals such as Ag, Cu, or Ni to Sn—Zn (see Japanese Published Unexamined Patent Applications Hei 8-267270, 9-94687, and 9-94688).
A Sn—Zn based lead-free solder having a composition close to a Sn—Zn eutectic composition has a liquidus temperature of around 200° C., and the temperature difference from the melting point of a Pb-63Sn eutectic solder is 17° C. Therefore, when performing soldering with a Sn—Zn based lead-free solder of electronic parts which from a temperature standpoint can just barely be soldered with a Pb-63Sn eutectic solder, this temperature difference of 17° C. (the soldering temperature is 17° C. higher) can be a problem. For this reason, there is sometimes a demand for a lead-free solder having a composition close to a Sn—Zn eutectic composition and having a liquidus temperature close to the melting point of a Pb-63Sn eutectic solder. Accordingly, there have been proposals of Sn—Zn based lead-free solders to which Bi or In, each of which has the effect of lowering the melting point of solder, has been added (see Japanese Patent No. 2914214, and Japanese Published Unexamined Patent Applications Hei 8-19892, 8-323495, and 9-19790).